1. Field of the Invention
The present invention relates to an injection molding machine, and more particularly to an injection molding machine having a sensor for detecting an amount of resin in a resin supplying hole that supplies resin to a heating cylinder.
2. Description of the Related Art
An injection molding machine repeatedly executes the steps of rotating a plasticizing screw inside a heating cylinder to knead and melt resin supplied to the interior of the heating cylinder and further metering the resin, then stopping screw rotation, advancing the screw and injecting melted resin into a mold, compressing, cooling and then opening the mold to remove the molded article.
The resin pellets that are put into the heating cylinder are dried in a drier and put into a hopper by a resin feeder or the like, pass through a resin supplying hole mounted on the machine main unit or the heating cylinder, ground by a revolving screw and melted. As a result, as the length of time the resin pellets remain inside the hopper and the resin supplying hole increases, the temperature of the resin pellets drops and the pellets collect moisture, which can cause molding defects. Consequently, in order to reduce temperature drop and humidification, and further, to maintain a uniform resin pellet density when the resin pellets are ground by the screw, only the minimum necessary amount of resin pellets are put into the hopper, the amount of resin pellets inside the hopper is measured by a resin amount detection device, and the amount of resin supplied to the hopper is adjusted to control the amount of resin remaining in the resin supplying hole, that is, the remaining resin amount (the resin deposit level).
Normally, the resin amount detection device of the resin feed machine is mounted above a hopper mounting port provided either at the base of the heating cylinder or the machine main unit. However, with this method, it is possible to detect the resin pellets only when the topmost surface of the resin is on the upstream side of the machine main unit, and moreover, it is difficult to mount the detection device near the screw. Accordingly, as described below, a variety of resin detection methods have been proposed.
Thus, an arrangement is known in which a capacitance proximity switch capable of moving along the outside of a resin supply cylinder that supplies resin to the resin supplying hole of the heating cylinder is provided and the remaining resin amount at a resin inlet is measured (for example, JP 3-121813U). However, because this arrangement places the sensor on the outside of the resin supply cylinder, it is difficult to place the sensor near the screw, and thus it is difficult to measure the remaining resin amount.
An arrangement in which a transparent portion formed from glass or the like is provided on one part of the resin supply cylinder that supplies resin to the resin supplying hole and a sensor at the position of the transparent portion measures the resin level inside the resin supply cylinder has also been proposed (for example, JP 63-94706U and JP 2003-236885A). However, with this method, to detect the resin level with the sensor it is necessary to make a portion of the resin supply cylinder out of glass or the like, which raises the cost of production. Further, it is to be expected that a complicated structure is necessary to move the sensor close to the screw and that it is therefore difficult to measure the resin pellet deposit level near the screw.
In addition, a device that disposes a light-emitting element and a light-receiving element and detects the resin deposit level has also been proposed (for example, JP 61-77220U). In this device, the sensor detection position is fixed and the resin amount to be detected cannot be changed.
Further, a method has been proposed that detects the resin pellet deposit level of the resin supply part with a reflective-type level sensor or photo sensor from the top of the screw (JP 3012320U). The remaining resin pellets are not always still due to the action of the screw and sometimes the resin pellets rebound due to the action of the screw. As a result, with this method, since the sensor detection surface faces in the direction of the screw, resin pellets thrown up by the action of the screw frequently strike the sensor detection surface, dirtying and scuffing the sensor detection surface, with the risk of easy deterioration of the sensor detection capability. If the sensor is moved away from the screw in order to prevent the dirtying of the sensor by the resin pellets thrown up by the action of the screw, the sensor is also removed from the topmost surface of the remaining resin, necessitating a high-accuracy sensor in order to measure the resin amount or for slightly changing the resin pellets deposit level.
In addition, since the resin deposit level must be measured by measuring the distance from the sensor to the resin, erroneous detection readings can easily occur when the resin is agitated by the action of the screw, and it is predicted that a high-accuracy sensor is required to prevent such erroneous measurement readings.